Fluid flowmeters of the pressure drop or differential pressure type are well known. Orifice plate and venturi flowmeters are probably the most common of the pressure drop type. Orifice plate flowmeters are inexpensive, but they are inherently high energy loss devices since the measured pressure drop across the orifice is non-recoverable, i.e. the drop in pressure is a drop in total pressure. Venturi flowmeters are low energy loss devices relative to orifice plate flowmeters since most of the pressure drop in the venturi throat is recoverable at the venturi outlet, i.e. the drop in pressure in the throat is due to an increase in kinetic energy of the fluid. However, when either of these flowmeters are used to measure fluid flow which varies over a wide range, such as air flow to an automotive engine, they either overly restrict total air flow at high engine speeds and loads if they are sized small enough to provide an adequate differential signal at low engine speeds and loads, or they provide an inadequate differential pressure signal at low engine speeds and loads if they are sized larger.
One prior art patent (U.S. Pat. No. 3,889,536) proposed a fluid flowmeter having a main air flow passage and a relatively small venturi in the main passage for receiving a portion of the total air flow and providing a static pressure signal for determining volumetric air flow in conjunction with a stagnation pressure signal in the main passage. This same patent also proposed placing a restriction in the main passage between the venturi inlet and outlet to increase the pressure difference across the venturi and thereby increase the pressure difference between the static and stagnation pressure. However, the restriction has the disadvantage of increasing the total pressure drop across the flowmeter, thereby increasing energy losses and decreasing the operating range of the flowmeter.
An additional shortcoming of many prior art devices resides in the fact that they are dedicated to a particular application and cannot easily be adjusted or reconfigured to accommodate differing applications or operational variations in a given application from system to system. For example, many flowmeters intended for automotive application are designed for an engine of known displacement and idealized respiration characteristics. Such flowmeters are often unsuitable for engines of slightly differing displacement or engines of the same displacement which fall in the outer fringe of design tolerances.
One prior art approach to effectively increase the operating range of a flowmeter while maintaining an acceptable pressure signal level is the bypass, which operates to shunt some of the fluid flowing through the meter around the swirl vanes, orifice, venturi or other signal generating element therein. Although such devices extend the range of operation, they have two major shortcomings. First, an inherent error factor is invited when the totality of air flow is not measured inasmuch as the ratio of measured air flow to bypassed air flow may vary. Additionally, such devices add mechanical complexity with its incumbent cost, reduced response (due to the mass of the moving parts) and shortened lifetime (due to the exposure of the interface between moving and non-moving parts to moisture and contaminants within the air flow). Finally, the use of pneumatic sensing techniques, although enjoying some commercial success, suffers from the frailties of requiring sensing ports which can become blocked by foreign matter and may have an unacceptably short useful life.
Another prior art approach which overcomes some of the shortcomings of pneumatic sensing is the use of hot wire anemometers or the like. Such devices operate by presenting a temperature dependent resistive element to the fluid flow and passing a current therethrough. The cooling effect of the fluid as it impinges upon the sensor is offset by modulation of the control voltage or current to maintain the resistor at a constant temperature. The variation of voltage or current is a measure of air flow. A substantial amount of literature has appeared recently relating to such techniques as reflected in many prior art patents.
The hot wire anemometer, however, has several shortcomings of its own. One shortcoming is lack of accuracy. Because the heated element is fixed within a fluid passageway and the velocity inlet profile of the fluid varies substantially with various operating conditions, prior art units were forced to tolerate nominal or compromise arrangements which built in error to the fluid flow measuring process. A related problem was in the fact that the temperature sensing elements were fixed within the passageway and could not be readily calibrated once the unit was fully assembled. The most serious shortcoming, however, was in the inherent fragile nature of the flow sensing elements and their tendency to collect contaminates on the surface thereof. Prior art designs, in order to improve response characteristics of the device attempted to minimize the thermal mass of the sensing element by making it extremely fine. Although successful laboratory tests were achieved, the application of such devices in a relatively hostile automotive environment where foreign particle matter pass through the meter at high velocity led to catastrophic failure of the device by breakage of the sensing element as well as reduced heat transfer (and thus inaccuracies) from contamination. The elements were also extremely sensitive to engine backfire.